How Distinctive Must a Modus Operandi Be to Serve as a "Signature"?

How distinctive does the manner of the commission of a crime have to be if that crime is to serve as a quasi-signature that tends to identify the perpetrator of that crime as the perpetrator of another crime committed in the same or similar manner? Cases and commentators once hinted the that m.o. used in the crime used as evidence of identity had to be unique, like a handwritten signature or a fingerprint, things that, some people (mistakenly) think, are unique identifiers. But today the law knows better: it knows that methods of committing crimes are not unique identifiers; today the law of evidence "knows" that methods of committing crimes can have at most only a tendency to show who the perpetrator of another crime was and the law now knows that an m.o. cannot be conclusive proof of a perpetrator's identity. But then the question is, "Well, just how unusual or distinctive must a modus operandi be if it is to be distinctive enough to allow "other bad act or crime" evidence to be admitted to show identity and to thus avoid falling afoul of the prohibition against the use of other crimes to show the commission of the crime charged?" The facts of some cases suggest that the answer increasingly is: not terribly different, the crime used as evidence of the identity of the perpetrator of the crime for which accused is on trial doesn't have to have been committed a very unusual way. See, e.g., United States v. Sappe, 898 F.2d 878, 879 (2d Cir. 1990), with my thanks again to Joannes Untalan Vinarao-Pilapil for finding this case.

There is a logic at work here: the logic of so-called logical relevance, which treats evidence as relevant if it has the slightest amount of probative value, if it has any tendency to increase or decrease the probability of a legally-material fact. See Federal Rule of Evidence 401. The same logic is corroding the doctrine of chances and the habit rule. In the case of the doctrine of chances, it is not apparent why more than just other other prior "accidental" bathtub drowning or more just one other supposed SIDS death is necessary to make the other drowning or the other infant death admissible in, respectively, a prosecution of accused for deliberately drowning his or her spouse or a father or mother for deliberately smothering an infant. And in the case of habit, it is hard to explain, under this logic (the logic of "logical relevance"), why a pattern of behavior must be invariable or almost invariable if the that pattern of behavior is to constitute habit legally speaking and thus avoid the prohibition against the use of character to show conduct.

It is this same sort of logic, I think, that drove the Supreme Court of the United States to hold that "clear and convincing" evidence of the commission of another crime is not necessary if another crime is to be used for a "nonpropensity purpose" such as "knowledge" or "absence of mistake" and that evidence "sufficient to support a finding" of the commission of the other crime is sufficient for admissibility of other crime evidence in federal trials. See Huddleston v. United States, 485 U.S. 681 (1988). The problem with this tendency (if you think it is a problem) is that if continued, it leads to the annihilation of all rules of evidence whose reason for being rests on their capacity to increase the accuracy of inference because of the accuracy of the generalizations those rules harbor about nature and humanity.

When I think about such questions, I tend to think that the fundamental questions about the reason for the existence of many rules of evidence have barely been posed, let alone answered.

the dynamic evidence page

coming soon: the law of evidence on Spindle Law